Soil gas contamination related to volatile organic and inorganic materials is common and can cause serious harm to people, other animals and plants. The United States spends billions of dollars each year treating soil gas contamination. Sources of soil gas contamination include leaking underground storage tanks, dry cleaners, leaking pipeline, chemical spills, landfills and transportation mishaps.
The vadose zone, between the ground surface and the water table, generally consists of subsurface sediments that were deposited in substantially horizontal layers. Soil gases exist in the subsurface layers often at pressures slightly higher than atmospheric pressure due to microbial respiration, physiochemical influences on contaminants, periodic decreases in barometric pressure, and increases in the partial pressure of organic compounds. Porosity varies from layer to layer, and some layers are impermeable, so that the conductivity of fluids is significantly greater horizontally than vertically. Impermeable surface covers, such as a concrete barrier, will also prevent upward migration of soil gas.
Soil Vapor Extraction (SVE) is one known method for treating soil gas contamination in the vadose zone. SVE systems can include injection wells with blowers that push air down the injection wells, spaced extraction wells with pumps that pull soil vapor and air out of the extraction wells, and air/water separators connected to the pumps. Soil vapor in porous subsurface layers migrates from higher pressure zones to lower pressure zones, so that the soil gas migrates horizontally into the extraction wells.
The injection and extraction wells in SVE systems are generally engineered wells in order to efficiently utilize the blowers and pumps. These engineered wells have a casing in a borehole. The casing has a smaller diameter than the borehole, and extends from the bottom of the borehole to a few feet above the ground level. The casing has a screen section that extends upwardly a selected distance from the bottom end. The well has porous fill material between the casing and the borehole wall from the bottom end up to slightly above the screen section, and a substantially impermeable material filling the space between the casing and the borehole wall from the porous fill material to the ground level, so that soil gas can only travel up the inside of the casing.
Installation of the injection and extraction wells for SVE systems can be relatively expensive. Power to run the blowers and pumps can be a major cost for SVE systems. Maintenance of the blowers and pumps adds to the ongoing costs of SVE systems. Regulatory requirements in many states require these engineered wells be removed after the wells are no longer in use, adding additional cost.
Passive Soil Vapor Extraction (PSVE) systems eliminate the use of conventional power sources to power the blowers and pumps. One known type of PSVE system uses wind or solar power to power the blowers and pumps. The second known type of PSVE system uses one way valves on the injection and extraction wells, and relies on variations in atmospheric barometric pressure to extract soil vapors. Both types of PSVE system use engineered wells with the same costs for construction, maintenance and removal costs. Although the pressure required to open the one way valves used in the second type of PSVE system is very low, the required pressure is still generally greater than normal diurnal barometric variations, so that these valves will only open during meteorological low pressure events. The one way valves used in the second type of PSVE system are prone to sticking due to insects, dirt, ice, or other materials, and therefore require maintenance.
The vadose zone has lots of small scale porous intervals, especially closer to the ground surface. The engineered wells in the known SVE and PSVE systems can treat only the single zone adjacent to the screen section at the bottom of the casing. The engineered wells in the known SVE and PSVE systems project above the ground layer and are therefore limited in location.